Configuration of overhead channels in a mixed bandwidth system

ABSTRACT

A method and apparatus for transmitting broadcast information in a multi-carrier communication system. The Sync Channel of the multi-carrier system is transmitted a 1.25 MHz channel bandwidth (i.e., over a single carrier), and to specify the preferred channels for the Sync Channel transmission instead of the preferred channels for the entire multi-carrier system. The Sync Channel Message will carry additional information indicating the center frequency of a multi-carrier system within a reserved set of frequency bands and indicating the frequency of a single carrier system in the reserved set of frequency bands. Considering the A block of the PCS band again, the preferred channels for Sync Channel transmission can be selected as channels 75, 150 and 225. This selection ensures that one of the preferred channels will always be used by any multi-carrier system regardless of the location of its center channel.

[0001] CLAIM OF PRIORITY UNDER 35 U.S.C. §120

[0002] The present Application for Patent is a Continuation and claimspriority to patent application Ser. No. 09/298,798 entitled“Configuration of Overhead Channels in a Mixed Bandwidth System” filedApr. 23, 1999, and assigned to the assignee hereof and hereby expresslyincorporated by reference herein.

BACKGROUND OF THE INVENTION

[0003] I. Field of the Invention

[0004] The present invention relates to communications. Moreparticularly, the present invention relates to a novel and improvedmethod and apparatus for transmitting and receiving broadcastinformation in a multi-carrier CDMA communication system.

[0005] II. Description of the Related Art

[0006] The use of code division multiple access (CDMA) modulationtechniques is one of several techniques for facilitating communicationsin which a large number of system users are present. Other multipleaccess communication system techniques, such as time division multipleaccess (TDMA) and frequency division multiple access (FDMA) are known inthe art. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems. The use of CDMA techniques in a multipleaccess communication system is disclosed in U.S. Pat. No. 4,901,307,entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USINGSATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of thepresent invention, of which the disclosure thereof is incorporated byreference herein. The use of CDMA techniques in a multiple accesscommunication system is further disclosed in U.S. Pat. No. 5,103,459,entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMACELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the presentinvention, of which the disclosure thereof is incorporated by referenceherein. CDMA has been standardized by the Telecommunications IndustryAssociation in Interim Standards IS-95A and IS-95B, entitled “MobileStation-Base Station Compatibility Standard for Dual Mode SpreadSpectrum Systems” (referred to hereafter collectively as IS-95).

[0007] In IS-95 communications systems channels of informationtransmitted from a common base station are distinguished from oneanother by orthogonal spreading codes. Each channel is spread by aunique orthogonal spreading sequence. The channels that are transmittedby an IS-95 base station include, a pilot channel, a sync channel, atleast one paging channel and dedicated traffic channels. The pilotchannel is used to provide a phase reference for coherent demodulationof the other channels by mobile stations within the coverage area of thebase station. The sync channel carries overhead information such astiming information, pilot PN offset information and other informationthat allows the reception of the other overhead channels. The pagingchannel notifies mobile stations of mobile terminated calls directed tothe mobile station in the area. Dedicated traffic channels provideinformation directed to the user of a specific mobile station in thecoverage area of the base station.

[0008] In IS-95, when the base station sends a Sync Channel Message, itshall uses the fixed-length message format of Table 1 below: TABLE 1Field Length (bits) MSG_TYPE (‘00000001’) 8 P_REV 8 MIN_P_REV 8 SID 15NID 16 PILOT_PN 9 LC_STATE 42 SYS_TIME 36 LP_SEC 8 LTM_OFF 6 DAYLT 1PRAT 2 CDMA_FREQ 11

[0009] MSG_TYPE Message type.

[0010] P_REV Protocol revision level.

[0011] MIN_P_REV Minimum protocol revision level. The base station setsthis field to prevent mobile stations which cannot be supported by thebase station from accessing the system.

[0012] SID System identification. The base station shall set this fieldto the system identification number for this system.

[0013] NID Network identification. This field serves as a sub-identifierof a system as defined by the owner of the SID.

[0014] PILOT_PN Pilot PN sequence offset index. The base station shallset this field to the pilot PN sequence offset for this base station, inunits of 64 PN chips.

[0015] LC_STATE Long code state. The base station shall set this fieldto the long code state at the time given by the SYS_TIME field of thismessage.

[0016] SYS_TIME System time. The base station shall set this field tothe System Time as of four Sync Channel super frames (320 ms) after theend of the last super frame containing any part of this Sync ChannelMessage, minus the pilot PN sequence offset, in units of 80 ms.

[0017] LP_SEC The number of leap seconds that have occurred since thestart of System Time.

[0018] LTM_OFF Offset of local time from System Time. The base stationshall set this field to the two's complement offset of local time fromSystem Time, in units of 30 minutes.

[0019] DAYLT Daylight savings time indicator. If daylight savings timeis in effect, the base station shall set this field to ‘1’; otherwise,the base station shall set this field to ‘0’.

[0020] PRAT Paging Channel data rate. The base station shall set thisfield to the PRAT field value shown in Table 2 corresponding to the datarate used by the Paging Channels in the system. TABLE 2 Paging ChannelData Rate PRAT Field (binary) Paging Channel data rate 00 9600 bps 014800 bps 10 Reserved 11 Reserved

[0021] CDMA_FREQ Frequency assignment. The base station shall set thisfield to the CDMA Channel number corresponding to the CDMA frequencyassignment for the CDMA Channel containing a Primary Paging Channel.

[0022] In IS-95 systems, each base station transmits a pilot channelthat is covered only by a short PN sequence. In IS-95 systems, the shortPN sequence repeats once every 26 ms. The pilot signal transmissionsfrom each base station are distinguished from one another by a phaseoffset with respect to one another. In particular, there each basestation associated with a single base station controller differs inphase by at least 64 PN chips.

[0023] Under normal operation, the mobile station acquires the pilotsignal first. The pilot signal carries no data and is simply the allones sequence spread by the common short code, which also spreads all ofthe other channels transmitted by the base station. After acquiring thepilot channel, the mobile station receives the information describedabove from the sync channel. The frame and interleaver timing on thesync channel are aligned with the pilot PN sequence. The zero state ofthe short PN sequence marks the starting point of the sync channel frameand interleaver.

[0024] In the U.S. Personal Communication System (PCS) spectrum, theCDMA channel number N specifies the carrier frequencies of the forwardand reverse link channels . In particular, channel number N correspondsto a reverse link carrier frequency of (1850+0.05N) MHz and a forwardlink carrier frequency of (1930+0.05N) MHz, where N ranges from 0 to1199. The bandwidth of each CDMA channel is 1.25 MHz. Therefore, thechannel numbers of adjacent CDMA channels are different by at least 25(25 58 0.05 MHz=1.25 MHz). To facilitate mobile stations' initialacquisition, certain carrier frequencies are designated as the preferredfrequency assignments. Referring to FIG. 1, for an IS-95 B CDMA systemin the A block of PCS band, the channel numbers of the preferredfrequency assignments are 25, 50, 75, 100, 125, 150, 175, 200, 225, 250,and 275. Mobile stations will search the preferred frequency assignmentsfirst upon powering up.

[0025] The International Telecommunications Union recently requested thesubmission of proposed methods for providing high rate data andhigh-quality speech services over wireless communication channels. Afirst of these proposals was issued by the Telecommunications IndustryAssociation, entitled “The cdma2000 ITU-R RTT Candidate Submission”(hereafter cdma2000). cdma2000 proposes increasing the throughput of theforward link signals by transmitting portions of the information onthree bands, each of 1.2288 MHz. This method is described as the“multi-carrier” approach.

[0026] A multi-carrier CDMA system, that uses three adjacent 1.25 MHz RFchannels with channel spacing also equal to 1.25 MHz is defined incdma2000. Referring to FIG. 2, a multi-carrier CDMA system in the Ablock of the PCS band can be deployed with the center channel on channel50, 75, 100, 125,150, 175, 200, 225, or 250. Channels 50 and 250 areusually avoided to avoid interference to adjacent bands on the reverselink. In the proposed cdma2000 description, the reverse link may bedirect-spread with a chip rate of 3.6864 Mcps, which makes the emissionlimit requirements more difficult to meet than for a chip rate of 1.2288Mcps.

[0027] A mobile station, upon powering up, searches for pilot signal ona preferred frequency. If no pilot is found in the current channel, itchanges its channel and searches again. Once a pilot is acquired, themobile station demodulates the Sync Channel associated with that pilotto receive timing information, pilot PN offset, and other informationenabling its reception of the other overhead channels.

[0028] In a multi-carrier approach, one method for providing SyncChannel data would be to tri-partition the Sync Channel message andplace one third of the message on each of the three portions of themulti-carrier signal. If the Sync Channel of a multi-carrier system isspread over three channels, the mobile station will have to know theexact channels used by the system in order to demodulate the SyncChannel reliably. Since the exact channels are not known in advance, themobile station has to try numerous combinations to receive the SyncChannel message. Given the number of preferred channels, the time spenton such trials could be excessive and therefore degrade the mobilestation's initial acquisition time. Therefore, there is a need felt inthe art for a method that minimizes the mobile station's search time.

SUMMARY OF THE INVENTION

[0029] The present invention is a novel and improved method andapparatus for transmitting broadcast information in a multi-carriercommunication system. The proposed invention is to send the Sync Channelof the multi-carrier system in a 1.25 MHz channel bandwidth (i.e., overa single carrier), and to specify the preferred channels for the SyncChannel transmission instead of the preferred channels for the entiremulti-carrier system. The Sync Channel Message will indicate the centerfrequency for a multi-carrier system in the band if one exists and thefrequency of a single band system if one exists.

[0030] Considering the A block of the PCS band again, the preferredchannels for Sync Channel transmission can be selected as channels 75,150 and 225. This selection ensures that one of the preferred channelswill always be used by any multi-carrier system regardless the locationof its center channel. The mobile station, upon powering up, searches onthe preferred channels for Sync Channel first. Once the pilot signal isacquired on any of these channels, the mobile station demodulates theSync Channel on the channel as well. The mobile station learns from theSync Channel Message the location of a multi-carrier and a singlecarrier system in the band if either exists. It can be easily observedthat the number of channels to search and the number of hypotheses totry are significantly reduced by using the proposal in this disclosure.As a result, it improves the mobile station's initial acquisition time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0032]FIG. 1 is an illustrative band diagram for a PCS system for 1xcommunications systems;

[0033]FIG. 2 is an illustrative band diagram for a PCS system for 3xcommunications systems;

[0034]FIG. 3 is a flowchart illustrating the acquisition method of thepresent invention;

[0035]FIG. 4 is a block diagram illustrating the major elements in awireless communications system;

[0036]FIG. 5 is a simplified block diagram of a multi-carriertransmission system;

[0037]FIG. 6 is a block diagram of CDMA modulation system;

[0038]FIG. 7 is a simplified block diagram of a multi-carrier receiversystem; and

[0039]FIG. 8 is a block diagram of a CDMA demodulation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040]FIG. 1 illustrates a typical band diagram for a multi-bandcommunications system. In currently contemplated wireless communicationssystems, mobile station that is attempting to begin service will tune toeach possible frequency in a preferred set of frequencies and determinewhether a system is available at that frequency. In FIG. 1, the channelnumbers of the preferred frequency assignments are 25, 50, 75, 100, 125,150, 175, 200, 225, 250, and 275, which corresponds to bands 200 a, 200b, 200 c, 200 d, 200 e, 200 f, 200 g, 200 h, 200 i, 200 j and 200 k. Inthe exemplary embodiment, each of these bands is 1.25 Mhz wide and isused to carry an IS-95 CDMA transmissions.

[0041]FIG. 2 illustrates the possible center bands of a three componentmulti-carrier communication system as is contemplated in cdma2000 (alsoreferred to as IS-2000). In the multi-carrier communication system, themobile station tunes to each possible grouping of three adjacentchannels and attempts to receive the Sync Channel message. In thecurrent designs for multi-carrier systems the Sync Channel message willbe divided into three component parts with each component parttransmitted separately and simultaneously on a different carrier of themulti-carrier band. The mobile station first attempts to receive theSync Channel message on a multi-carrier system comprising bands 300 b,300 c, 300 d. If unsuccessful, the mobile station then attempts toacquire the Sync Channel on a multi-carrier system consisting of bands300 c, 300 d, and 300 e. This continues for each possible three bandsystem until the mobile station checks for a multi-carrier systemconsisting of bands 300 h, 300 i and 300 j. For reasons described above,in the exemplary embodiment, bands 300 a and 300 k will not be used in amulti-carrier.

[0042] This method of receiving the Sync Channel message is veryinefficient and time consuming. If a mobile station is capable ofoperating in a multi-carrier or single carrier mode, the mobile stationwill potentially have to conduct eleven 1x searches or bands 200 a -200k and seven multi-carrier searches using center frequencies 300 c -300i. In a CDMA communication system, each band searched requires themobile station to test a large number of PN offsets to detect thepresence of the pilot signal. Thus, this method of acquisition requiresa substantial time period.

[0043] The present invention provides a significantly more efficientmethod for acquiring the necessary system parameters in a potentiallymixed bandwidth communication system. In the present invention, the SyncChannel is always transmitted in a 1x band. In the preferred embodiment,the preferred channels are channels 75, 150 and 225. Thus, a mobilestation need only conduct at most three searches to receive the SynchChannel message in 1x bandwidth in order to acquire the necessaryinformation for acquisition of its preferred system. The presentinvention greatly reduces acquisition time in a mixed bandwidthcommunication system. In addition, by providing the Sync Channel messageonly on the preferred channels reduces the capacity impact of providingthat overhead messaging on many more channels.

[0044] The reason for the allocation of the preferred channels of thepresent invention is that it provides the greatest flexibility in theprovision of a multi-carrier system. With the allocation of thepreferred channels to channel numbers 75, 150 and 220 (300 c, 300 f and300 j), a multi-carrier system provided anywhere in the band consistingof bands 300 a -300 k will include one of the preferred channels. Amulti-carrier system comprising the bands 300 a, 300 b and 300 c willinclude the preferred channel 300 c. A multi-carrier system comprisingthe bands 300 b, 300 c and 300 d will include the preferred channel of300 c. A multi-carrier system comprising the bands of 300 c, 300 d and300 e will include the preferred channel of 300 c. A multi-carriersystem comprising the bands of 300 d, 300 e and 300 f will include thepreferred channel of 300 f. Any combinations of three adjacent bandswill include a preferred channel upon which the mobile station will beable to receive the necessary system parameters for operation.

[0045] In the present invention, the mobile station tunes to a preferredchannel (300 c, 300 f or 300 i) and attempts to detect a pilot signal onthat frequency band. If a pilot signal is detected the mobile stationreceives, demodulates and decodes the Sync Channel message. In thepresent invention, the Sync Channel message will provide informationthat identifies the center frequency of a multi-carrier system in thecurrent set of frequency bands (if one exists) and the frequency of a 1xband in the current set of frequency bands (if one exists).

[0046] The mobile station in response to the information received in theSync Channel selects the system that is appropriate for its needs orcapabilities. If the mobile station prefers to use a multi-carriersystem, the mobile station uses the center frequency of themulti-carrier system indicated in the Sync Channel message to tune tothe multi-carrier system and receive the broadcast channel (BCH)message. The broadcast channel message will indicate to the mobilestation the number of common control channels (CCCHs) that are employedby the current system. The mobile station takes the number of commoncontrol channels and using a predetermined hashing algorithm determineswhat code channel it will use to receive page messages from thetransmitting base station.

[0047] If the mobile station selects to operate in a single band system,the mobile station uses the information received in the Sync Channelmessage to tune to the appropriate frequency of the single band system.The mobile station then receives the general paging channel message onthe primary paging channel. The general paging channel message providesthe number of paging channels used by the single band system. The mobilestation uses a predetermined hashing function to determine the codechannel upon which the mobile station will receive paging messages forthe transmitting base station.

[0048] The present invention is equally applicable to systems that mayinclude 3X direct spread communications systems. In this embodiment, theSync Channel message would include the additional information whetherthe 3X system is a direct spread system or a multi-carrier system. Inaddition, the Sync Channel message may also provide informationrespecting whether the system uses a form of diversity transmission suchas orthogonal transmit diversity (OTD). If methods of transmissiondiversity are possible in the multiple bandwidth communication system,specification of the means of transmit diversity greatly reduces thenumber of hypotheses that must be tested to acquire a system.

[0049] These changes to the Sync Channel messages can be accommodatedwithout necessitating the extension of the Sync Channel message used inIS-95B and illustrated above. In the current Sync Channel message therea large number of reserved bits that can be used to provide theadditional information.

[0050]FIG. 3 is a flowchart illustrating the acquisition operation ofthe present invention. In block 2, the mobile station tunes to apreferred channel (300 c, 300 f or 300 i). It will be understood thatthe present invention is disclosed in context of the PCS band offrequencies and can easily be extended to other bands such as thecellular band. Moreover, the selected set of preferred channels thoughpreferred for three carrier, multi-carrier communications systems wouldbe different for multi-carrier systems with a different number ofcarriers.

[0051] In block 6, the mobile station determines whether the searchoperation was successful. In the exemplary embodiment, the presentinvention is incorporated into a CDMA communications system, though thepresent invention would be equally applicable to other mixed bandwidthcommunications systems. In the exemplary embodiment, the mobile stationtunes its RF receiver to a preferred channel (300 c, 300 f, or 300 i)and attempts to detect the presence of a pilot signal. In the exemplaryembodiment of an IS-95 based CDMA communications system, each basestation transmits its pilot signal using a unique pilot signal offset.Thus, the mobile station after tuning to a preferred channel (300 c, 300f, or 300 i), searches the possible PN offset hypotheses.

[0052] The method and apparatus for detecting a pilot signal in a CDMAcommunication system are well known in the art and are described indetail in U.S. Pat. No. 5,644,591, entitled “Method And Apparatus ForPerforming Search Acquisition In A CDMA Communication System”, which isassigned to the assignee of the present invention and incorporated byreference herein. The mobile station tests each PN offset hypothesis bycomputing the correlation between the received signal within thepreferred frequency band with the PN hypothesis being tested. If thecorrelation energy for all PN hypotheses is less than a threshold value,then the acquisition at the preferred frequency is not successful andthe operation moves to block 4. In block 4, the next preferred channel(300 c, 300 f, or 300 i) to be tested by the mobile station is selectedand the flow moves to block 2 and continues as described.

[0053] When the mobile station detects sufficient correlation energybetween the received signal at the preferred channel frequency and a PNoffset hypothesis, a successful acquisition is declared. At this pointthe operation moves to block 8. In block 8, the mobile station receivesthe Sync Channel message. In the present invention, the Sync Channelmessage is transmitted in a single 1x band (300 c, 300 f, or 300 i). Inthe exemplary embodiment of an IS-95 based CDMA communication system,the Sync Channel frame boundary and the interleaver boundary are alignedwith the short PN sequence used to spread the pilot channel signal.Thus, upon successful acquisition of the pilot channel signal, themobile station has enough information to de-interleave and decode theSync Channel message.

[0054] After receiving the Sync Channel message, the operation moves toblock 10. The mobile station determines from the Sync Channel messagethe center frequency of a multi-carrier system in the current set ofbands (if one exists) and the location of a single carrier band in thecurrent set of bands (if one exists). The mobile station decides whetherto operate in a multi-carrier mode or a single channel mode based on itscapabilities and needs.

[0055] The operation moves to control block 10. If the mobile stationcapable of operating in a multi-carrier mode decides to operate inmulti-carrier mode, then the flow moves to block 12. In block 12, themobile station initiates its RF hardware for multi-carrier reception.The mobile station will know from the received Sync Channel message thecenter frequency of a multi-carrier system in the current set offrequency bands if one exists. The operation then moves to block 14,where the mobile station receives the broadcast channel (BCH) signal andfrom that channel learn among other information the number of commoncontrol channels used by the communication system. The mobile stationhashes the number of common control channels to determine the codechannel that it should use to receive pages.

[0056] If, back in block 10, the mobile station decides to use a singlechannel system because of limitations in its capability or otherpreference with respect to its service needs, the operation moves toblock 16. In block 16, the mobile station initiates its RF hardware forsingle-carrier reception. The operation then moves to block 18, wherethe mobile station receives the general page message on a predeterminedcode channel. The general page message will state the number of pagingchannels used by the system. The mobile station hashes the number ofpaging channels used to determine the code channel that it will use toreceive directed pages from the serving base station.

[0057]FIG. 4 introduces the elements and nomenclature of a verysimplified wireless communications system. Base station 30 transmitsforward link signal 32 to mobile station 40. Mobile station 40 transmitsreverse link signal 34 to base station 30.

[0058]FIG. 5 is a simplified block diagram illustrating the exemplaryembodiment of base station 30 as multi-carrier CDMA transmission systemwith three forward link channels. Each of transmission subsystems 48transmits a portion of forward link signal 32 on a different carrierfrequency. Transmission subsystem 48 a transmits a portion of forwardlink signal 32 on frequency f1, transmission subsystem 48 b transmits aportion of forward link signal 32 on frequency f2, and transmissionsubsystem 48 c transmits a portion of forward link signal 32 onfrequency f3.

[0059] The data for transmission on forward link signal 32 is providedto de-multiplexer 50. De-multiplexer 50 provides the data to one of thethree transmission subsystems 48. The present invention is described interms of a three carrier multi-carrier communication system becausethree IS-95 carriers each occupying 1.2288 MHz can fit into a 5 MHzband. However, it will be understood by one skilled in the art that theteachings of the present invention can easily be extended to anarbitrary number of channels in a multi-carrier system.

[0060] The de-multiplexed data streams are provided to modulators 52. Inthe exemplary embodiment, modulators 52 modulate the forward link datain accordance with a CDMA modulation format such as that described inthe IS-95 standard and described in detail in the aforementioned U.S.Pat. No. 5,103,459. The forward link data includes dedicated channeldata that is for provision to a specific mobile station and broadcastchannel data that is for provision to all mobile stations in thecoverage area of base station 30 or a subset of mobile stations 40 inthe coverage area of base station 30. The Sync Channel message is anexample of broadcast data that is transmitted to all mobile stations inthe coverage area of base station 30. In the present invention, the SyncChannel message is provided to a designated one of transmissionsubsystems 48 for transmission on a designated one of the threecarriers.

[0061] The modulated forward link data from modulators 52 is provided toup converter 54. Up converter 54 up coverts the modulated signal to acarrier frequency (f1, f2, or f3) that is generated by a localoscillator (not shown). The up converted signals are then combined fortransmission through antenna 56.

[0062]FIG. 6 illustrates the exemplary embodiment of modulators 52 whichmodulate a portion of the forward link signal for transmission on asingle carrier of forward link signal 32. In the exemplary embodiment, apilot signal is transmitted to allow for coherent demodulation of thesignal by the receivers, which improves the performance of the receiverby providing a phase reference for demodulation. A set of pilot symbolsknown to both base station 30 and mobile station 40 are provided toWalsh spreader 60. Walsh spreader 60 spreads the pilot symbols by aWalsh sequence Wpilot. In the exemplary embodiment, Walsh sequences areused to distinguish the channels of the data transmitted on a singleCDMA carrier. The Walsh function can be either of a fixed number ofsymbols as is described in the IS-95 specification or it can be anorthogonal function that varies in length in accordance with the rate ofthe data to be transmitted on the channel as is described in thecdma2000 proposal and in U.S. Pat. No. 5,751,761, entitled “System andMethod for Orthogonal Spread Spectrum Sequence Generation in VariableData Rate Systems”, which is assigned to the assignee of the presentinvention and incorporated by reference herein.

[0063] The Walsh spread pilot symbols are provided to complex PNspreader 62. Complex PN spreader 62 spreads the Walsh spread pilotsymbols in accordance with two separately generated pseudonoise (PN)sequences, PNI and PNQ. If the two inputs to complex PN spreader 62 aredesignated as I and Q, the result of the complex spreading operation aretwo channels I′ and Q′ given by the equations:

I′=PN _(I) I−PN _(Q) Q  (1)

Q′=PN _(Q) I+PN _(I) Q.  (2)

[0064] The purpose of the complex PN spreading is to more evenlydistribute the loading on the in-phase and quadrature channels of theQPSK modulator, which results in a reduction of the peak to averageratio on the power amplifier (not shown) of base station 30 which inturn increases the capacity of base station 30. Complex PN spreading isdescribed in the cdma2000 RTT proposal and is described in detail incopending U.S. patent application Ser. No. 08/886,604, entitled “HighData Rate CDMA Wireless Communication System”, which is assigned to theassignee of the present invention and is incorporated by referenceherein. The complex PN spread pilot symbols are provided to transmitter(TMTR) 94, which up converts, filters and amplifies the signal fortransmission through antenna 56.

[0065] In the exemplary embodiment, the Sync Channel message isdistinguished from other channels of information by being spread by aunique orthogonal spreading sequence Wsync. In the preferred embodimentof the present invention, the sync channel message is only transmittedby a selected one of modulators 48 a, 48 b or 48 c. The selectedmodulator 48 transmits the Sync Channel message on a preferred channel.In the exemplary embodiment, the Sync Channel message indicates thecenter frequency of a multi-carrier system in the current band offrequencies, if one exists, and the frequency of a single carrier systemin the current set of frequency bands, if one exists.

[0066] The Sync Channel message is provided to message formatter 64. Inthe exemplary embodiment, message formatter 64 generates a set of cyclicredundancy check (CRC) bits and an optional set of tail bits and appendsthose bits to the Sync Channel message. IS-95 systems do not append tailbits to the Sync Channel message. cdma2000 (also referred to as IS-2000)systems append eight tail bits to the Sync Channel message. The Syncchannel message with the CRC bits and tails bits appended is provided toencoder 66. Encoder 66 encodes the sync channel message, CRC bits andtail bits in accordance with a predetermined forward error correctioncoding algorithm such as convolutional coding.

[0067] The encoded symbols are then provided to interleaver (INT) 68which reorders the encoded symbols in accordance with a predeterminedinterleaving format. The interleaver is provided to provide timediversity in the transmitted stream of encoded symbols. Decoders havebetter error correction performance when the errors in the receivedstream are not burst errors.

[0068] The reordered symbols are provided to Walsh spreader 70 whichspreads the reordered symbols in accordance with a predetermined codesequence Wsync. In the exemplary embodiment, Wsync is a code sequencethat is orthogonal to all other code sequences used to channelize thechannels of forward link signal 32. The Walsh spread signal is thenprovided to complex PN spreader 62 and is spread as described above.

[0069] Common channel messages are transmitted to all subscriberstations or sets of subscriber stations within the coverage area of basestation 30. Examples of common channel messages include paging messagesthat alert mobile stations of incoming calls and control channelmessages that provide necessary control information to mobile stationsin the coverage area of base station 30. For illustration purposes asingle control channel is shown. It will be understood by one skilled inthe art that in practical implementations a plurality of controlchannels will be transmitted a base station 30.

[0070] The Common Channel message is provided to message formatter 74.In the exemplary embodiment, message formatter 74 generates a set ofcyclic redundancy check (CRC) bits and a set of tail bits and appendsthose bits to the Common Channel message. The Common Channel messagewith the CRC bits and tails bits appended is provided to encoder 76.Encoder 76 encodes the Common Channel message, CRC bits and tail bits inaccordance with a predetermined forward error correction codingalgorithm such as convolutional coding.

[0071] The encoded symbols are then provided to interleaver (INT) 78which reorders the encoded symbols in accordance with a predeterminedinterleaving format. The interleaver is provided to provide timediversity in the transmitted stream of encoded symbols. Decoders havebetter error correction performance when the errors in the receivedstream are not burst errors.

[0072] The reordered symbols are provided to Walsh spreader 82 whichspreads the reordered symbols in accordance with a predetermined codesequence Wcc. In the exemplary embodiment, Wcc is a code sequence thatis orthogonal to all other code sequences used to channelize thechannels of forward link signal 32. The Walsh spread signal is thenprovided to complex PN spreader 62 and is spread as described above.

[0073] Dedicated Channel data is transmitted to a particular subscriberstation within the coverage area of base station 30. The DedicatedChannel data is provided to message formatter 84. In the exemplaryembodiment, message formatter 84 generates a set of cyclic redundancycheck (CRC) bits and a set of tail bits and appends those bits to theframe of dedicated channel data. The frame of Dedicated Channel datawith the CRC bits and tails bits appended is provided to encoder 86.Encoder 86 encodes the frame of Dedicated Channel data, CRC bits andtail bits in accordance with a predetermined forward error correctioncoding algorithm such as turbo coding or convolutional coding.

[0074] The encoded symbols are then provided to interleaver (INT) 88which reorders the encoded symbols in accordance with a predeterminedinterleaving format. The interleaver is provided to provide timediversity in the transmitted stream of encoded symbols. Decoders havebetter error correction performance when the errors in the receivedstream are not burst errors.

[0075] The reordered symbols are provided to Walsh spreader 90, whichspreads the reordered symbols in accordance with a predetermined codesequence WT. In the exemplary embodiment, WT is a code sequence that isorthogonal to all other code sequences used to channelize the channelsof forward link signal 32. The Walsh spread signal is then provided tocomplex PN spreader 62 and is spread as described above.

[0076] The complex PN spread data is provided to transmitter (TMTR) 94and up converted, filtered and amplified for transmission throughantenna 56.

[0077] Turning to FIG. 7, a simplified multi-carrier receiver which inthe exemplary embodiment is provided in mobile station 40 isillustrated. In the exemplary embodiment, mobile station 40 is capableof simultaneously receiving forward link signals 32 transmitted on up tothree carrier channels. It will be understood by one skilled in the artthat the present invention can be extended to multi-carrier reception ofan arbitrary number of channels. The received signal is provided to eachof receive subsystems 105 which down convert and demodulate a differentcomponent of forward link signal 32 in accordance with a unique carrierfrequency.

[0078] The forward link signal 32 is received at antenna 100 andprovided to receivers 102. Each of receivers 102 a, 102 b and 102 c downconvert, filter and amplify the received signal in accordance with adifferent frequency f1, f2, or f3, respectively. The down convertedsignals are provided to demodulators 104. In the exemplary embodiment,demodulators 104 demodulate each of the down converted signals inaccordance with a code division multiple access (CDMA) modulationformat. The implementation of demodulators 104 is described in detail inthe aforementioned U.S. Pat. No. 5,103,459. The demodulated componentsof forward link signal 32 are provided to multiplexer (MUX) 106 whichre-assembles the transmitted data stream.

[0079] In the present invention, mobile station 40 initially only uses asingle receiver 102 and demodulator 104 to demodulate the Sync Channel.Mobile station 40 tunes a selected receivers 102 to a preferred channel(300 c, 300 f or 300 i) and attempts to acquire the pilot signal at thepreferred channel frequency using a corresponding one of demodulators104. If sufficient correlation energy is detected, the acquisition isdeclared to be successful. The mobile station then still only downconverting at the single frequency, demodulates, de-interleaves anddecodes the Sync Channel message. From the Sync Channel message, mobilestation 40 determines the center frequency of a multi-carrier system inthe current frequency bands, if one exists, and the frequency of asingle carrier system in the current frequency bands, if one exists.

[0080] Mobile station 40 decides whether it will operate in amulti-carrier mode or a single carrier mode. If mobile station 40decides to operate in a multi-carrier mode, then mobile station 40activates the RF circuitry additional receivers 102, tuning to theappropriate set of frequencies indicated in the Sync Channel message andbegins to receive the forward link signal on a plurality of carrierfrequencies. If mobile station 40 decides to operate in a single-carriermode, then mobile station 40 tunes to the appropriate frequencyindicated in the Sync Channel message and begins to receive the forwardlink signal on a single carrier frequency band.

[0081]FIG. 8 illustrates the apparatus to receive forward link signal 32in the exemplary embodiment of a CDMA communication system. Initially,mobile station 40 must tune to a preferred channel and attempt toacquire the pilot signal of that channel in the following manner.

[0082] The forward link signals 32 are received at antenna 100 andprovided to receiver 102. Receiver 102 tunes to a preferred channelfrequency and down converts, filters and amplifies the received signal.In the exemplary embodiment, receiver 102 is a quaternary phase shiftkeyed receiver and outputs an in-phase (I) and Quadrature (Q) componentof the received signal.

[0083] The two components of the received signal are provided to complexPN despreader 112. Complex PN despreader 112 despreads the receivedsignal in accordance with two pseudonoise sequences PNI and PNQ. In theexemplary embodiment, the PN despreading is a complex PN despreading asis described in detail in the aforementioned copending U.S. patentapplication Ser. No. 08/886,604. In the exemplary embodiment, the PNsequences used to spread forward link signals 32 are generated using agenerator polynomial that is common to all base stations 30. Thespreading from base stations is distinguished from one another by theoffset of the sequence.

[0084] Control processor 128 provides offset hypotheses to complex PNdespreader 112. Complex PN despreader 112 despreads the received signalsin accordance with the PN offset hypotheses provided by controlprocessor 128 as well as Wpilot. The received signal is despread inaccordance with the PN offset hypothesis and the resultant signal isprovided to pilot filter 114. Pilot filter 114 despreads the signal fromcomplex PN despreader 112 in accordance with an orthogonal sequenceWpilot and low pass filters the result of complex despreader 112. In theexemplary embodiment, the Walsh sequence comprising of all 1's is usedto channel the pilot signal.

[0085] The resultant signals from pilot filter 114 are provided toenergy detector 118 which sums the squares of the resultant samples frompilot filter 114 to provide a received pilot energy value. The receivedpilot energy value is provided to control processor 128, where it iscompared to a predetermined threshold value. If the computed energyexceeds the threshold acquisition is declared to be successful and themobile station begins to receive the Sync Channel message. If the energyfalls below the threshold value, then the acquisition is declaredunsuccessful and the next PN hypothesis is provided by control processor128 to complex PN despreader 112. The method and apparatus for searchingPN offsets in a CDMA communication system is described in detail in U.S.Pat. No. 5,644,591, entitled “Method And Apparatus For Performing SearchAcquisition In A CDMA Communication System”, which is assigned to theassignee of the present invention and incorporated by reference herein.If, after exhausting the possible PN offset hypotheses, the receivedpilot energy fails to exceed the threshold value, then control processorsends message to receiver 102 to begin down converting the receivedsignal at a different preferred frequency channel.

[0086] Upon successful acquisition of a pilot channel on a preferredchannel frequency, mobile station 40 demodulates and decodes the SyncChannel message. The received signal from receiver 102 is despread usingthe PN offset determined in the pilot search algorithm. The pilot signalis processed by pilot filter 114 as described above.

[0087] The PN despread signal is also provided to Walsh despreader 116.Walsh despreader 116 despreads the received signal in accordance with aWalsh code sequence Wchan. When demodulating the Sync Channel, the Wchanis Walsh sequence allocated for the transmission of the Sync Channelmessage. Walsh despreader 116 despreads the signal components inaccordance with the orthogonal sequence Wchan and provides the result todot product circuit 120.

[0088] As forward link signal 32 traverses the propagation path tomobile station 40, an unknown phase component is introduced into thereceived signal. Dot product circuit 120 computes the projection of thereceived signal onto the received pilot signal to provide a scalarresult without the phase errors. The implementation of dot productcircuits for coherent demodulation are well known in the art and amethod and apparatus for performing the dot product procedure isdescribed in detail in U.S. Pat. No. 5,506,865, entitled “Pilot CarrierDot Product Circuit” which is assigned to the assignee of the presentinvention and is incorporated by reference herein.

[0089] The scalar outputs from dot product circuit 120 are provided tomultiplexer 122 which combine the two received streams into a signaldata stream. The data stream from multiplexer 122 is provided tode-interleaver 124 which reorders the received symbols in accordancewith a predetermined de-interleaving format. The reordered symbols areprovided to decoder 126 which decodes the symbols of the Sync Channelmessage to provide the received Sync Channel message.

[0090] The received Sync Channel message is provided to controlprocessor 128. In the present invention, control processor 128determines from the Sync Channel message the channel frequency for theprimary overhead channel of the single carrier system or the centerfrequency of the multi-carrier system. In response to the Sync Channelmessage, control processor 128 initiates the correct number of receivesubsystems 105 and tunes them to the appropriate channels to receiveforward link signal 32.

[0091] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

I claim:
 1. A multi-carrier base station, comprising: a firsttransmission subsystem for transmitting some data components of forwardlink data comprising a sync channel message on at least one of a set ofpreferred frequency channels included within a predetermined set offrequencies in a personal communications system (PCS) block offrequencies ; and at least one additional transmission subsystem forsimultaneously transmitting remaining components of said forward linkdata.
 2. The base station of claim 1, wherein said sync channel messageindicates the frequency of a single carrier system in said set offrequencies.
 3. The base station of claim 1, wherein said sync channelmessage indicates the center frequency of at least one multi-carriersystem in said set of frequencies.
 4. The base station of claim 3,wherein said set of a preferred frequency channels have channel numberscomprising 75, 150 and 225, and wherein the number of frequency channelsin said set of preferred frequency channels is less number offrequencies in the predetermined set of frequencies.
 5. A method ofacquiring a sync channel message using a wireless device operable in atleast one of a single carrier mode in a single carrier system and amulti-carrier mode in a multi-carrier system, the method comprising:defining a predetermined set of frequencies that each have channelnumber associated with a band; and tuning to preferred channel numbersuntil a sync channel message is received, wherein any combination ofthree adjacent channel numbers includes a preferred channel number uponwhich the wireless device can receive the sync channel message.
 6. Amethod according to claim 5, wherein tuning to preferred channel numbersuntil a sync channel message is received, comprises: attempting toreceive the sync channel message by tuning to a first preferred channelnumber located on one of a group of bands including the first throughthird bands; and if the sync channel message is not received on thegroup of bands including the first through third bands, attempting toreceive the sync channel message by tuning to the first preferredchannel number located on one of a group of bands including the secondthrough fourth bands.
 7. A method according to claim 6, furthercomprising: if the sync channel message is not received on the group ofbands including the second through fourth bands, attempting to receivethe sync channel message by tuning to a second preferred channel numberlocated on one of a group of bands including the third through fifthbands.
 8. A method according to claim 7, further comprising: if the syncchannel message is not received on the group of bands including thethird through fifth bands, attempting to receive the sync channelmessage by tuning to the second preferred channel number located on oneof a group of bands including the fourth through sixth bands.
 9. Amethod according to claim 8, wherein channel numbers of thepredetermined set of frequencies comprise 25, 50, 75, 100, 125, 150,175, 200, 225, 250, and 275, and wherein the preferred channel numbersare 75, 150 and 225 such that such that a sync channel message providedanywhere in the predetermined set of frequencies will include one of thepreferred channel numbers.
 10. A method according to claim 9, whereineach band has a width of approximately 1.25 Mhz and carries CDMAtransmission.
 11. A method according to claim 5, wherein the syncchannel message is received if: a detector in the wireless devicedetects a pilot signal on a band that includes a preferred channelnumber; and a receiver in the wireless device receives, demodulates, anddecodes the sync channel message.
 12. A method according to claim 5,further comprising: deciding to operate, responsive to the sync channelmessage, in one of the multi-carrier system and the single carriersystem.
 13. A method according to claim 12, wherein the sync channelmessage is transmitted in a 1x band and indicates the frequency of a 1xband in the current set of frequency bands, and if the wireless devicedecides to operate in the single carrier system, then the wirelessdevice uses the information received in the sync channel message to tuneto the appropriate frequency of the single carrier system, and receivesa general paging channel message on a primary paging channel thatprovides a number of paging channels used by the single carrier system.14. A method according to claim 12, wherein the sync channel messageindicates the center frequency of a multi-carrier system in the currentset of frequency bands, and if the wireless device decides to operate inthe multi-carrier system, then the wireless device uses the centerfrequency of the multi-carrier system indicated in the sync channelmessage to tune to the multi-carrier system and receive a broadcastchannel message that indicates a number of common control channels thatare employed by the multi-carrier system.
 15. A method of acquiring async channel message using a wireless device operable in at least one ofa single carrier mode in a single carrier system and a multi-carriermode in a multi-carrier system, the method comprising: defining apredetermined set of frequencies that each have channel numberassociated with a band; and tuning to preferred channel numbers until async channel message is received, wherein any combination of threeadjacent channel numbers includes a preferred channel number upon whichthe wireless device can receive the sync channel, wherein tuning topreferred channel number until a sync channel message is received,comprises: tuning to a first preferred frequency in the predeterminedset of frequencies, wherein the first preferred frequency falls withinthe first though third bands; and determining whether a sync channelmessage is available in the first though third bands, wherein the syncchannel message is divided into three component parts that are eachtransmitted separately and simultaneously on a different bands; if thesync channel message is not available at the first preferred frequency,tuning to a second preferred frequency in the predetermined set offrequencies, wherein the second preferred frequency falls within thesecond through fourth bands; and determining whether the sync channelmessage is available in the second through fourth bands.
 16. A methodaccording to claim 15, wherein channel numbers of the predetermined setof frequencies comprise 25, 50, 75, 100, 125, 150, 175, 200, 225, 250,and 275, and wherein the preferred channel numbers are 75, 150 and 225such that such that a sync channel message provided anywhere in thepredetermined set of frequencies will include one of the preferredchannel numbers.
 17. A method according to claim 16, wherein each bandhas a width of approximately 1.25 Mhz and carries CDMA transmission. 18.A wireless device operable in at least one of a single carrier mode in asingle carrier system and a multi-carrier mode in a multi-carriersystem, comprising: means for storing a predetermined set of frequenciesthat each have channel number associated with a band; and means fortuning to preferred channel numbers until a sync channel message isreceived, wherein any combination of three adjacent channel numbersincludes a preferred channel number upon which the wireless device canreceive the sync channel message.
 19. A wireless device according toclaim 18, wherein the means for tuning to preferred channel numbersuntil a sync channel message is received, comprises: means forattempting to receive the sync channel message by tuning to a firstpreferred channel number located on one of a group of bands includingthe first through third bands, and, if the sync channel message is notreceived on the group of bands including the first through third bands,tuning to the first preferred channel number located on one of a groupof bands including the second through fourth bands.
 20. A wirelessdevice according to claim 19, if the sync channel message is notreceived on the group of bands including the second through fourthbands, wherein the means for attempting to receive the sync channelmessage tunes to a second preferred channel number located on one of agroup of bands including the third through fifth bands.
 21. A wirelessdevice according to claim 20, if the sync channel message is notreceived on the group of bands including the third through fifth bands,wherein the means for attempting to receive the sync channel messagetunes to the second preferred channel number located on one of a groupof bands including the fourth through sixth bands.
 22. A wireless deviceaccording to claim 21, wherein channel numbers of the predetermined setof frequencies comprise 25, 50, 75, 100, 125, 150, 175, 200, 225, 250,and 275, and wherein the preferred channel numbers are 75, 150 and 225such that such that a sync channel message provided anywhere in thepredetermined set of frequencies will include one of the preferredchannel numbers.
 23. A wireless device according to claim 22, whereineach band has a width of approximately 1.25 Mhz and carries CDMAtransmission.
 24. A wireless device according to claim 18, wherein thesync channel message is received if: a detector in the wireless devicedetects a pilot signal on a band that includes a preferred channelnumber; and a receiver in the wireless device receives, demodulates, anddecodes the sync channel message.
 25. A wireless device according toclaim 18, further comprising: means for deciding to operate, responsiveto the sync channel message, in one of the multi-carrier system and thesingle carrier system.
 26. A wireless device according to claim 25,wherein the sync channel message is transmitted in a 1x band andindicates the frequency of a 1x band in the current set of frequencybands, and if the means for deciding decides to operate in the singlecarrier system, then the wireless device uses the information receivedin the sync channel message to tune to the appropriate frequency of thesingle carrier system, and receives a general paging channel message ona primary paging channel that provides a number of paging channels usedby the single carrier system.
 27. A wireless device according to claim25, wherein the sync channel message indicates the center frequency of amulti-carrier system in the current set of frequency bands, and if themeans for deciding decides to operate in the multi-carrier system, thenthe wireless device uses the center frequency of the multi-carriersystem indicated in the sync channel message to tune to themulti-carrier system and receive a broadcast channel message thatindicates a number of common control channels that are employed by themulti-carrier system.